The answer is an MTU mismatch between the routers, as this is the most common cause of OSPF neighbors becoming stuck in the EXSTART state. In EXSTART, routers attempt to exchange Database Description (DBD) packets to establish a master/slave relationship and agree on an initial sequence number; if one router’s interface has a smaller MTU, it will silently drop DBD packets that exceed that size, halting the OSPF adjacency process before it can reach the Loading state. On the ENCOR 350-401 exam, this scenario tests your understanding of OSPF state progression and Layer 2 constraints—a common trap is to blame authentication or network type mismatches, but MTU mismatch is the primary culprit when neighbors stall specifically in EXSTART. To remember this, think of the mnemonic “EXSTART Exits when MTU is Too Small,” as the smaller MTU router effectively refuses oversized DBD packets, blocking the adjacency from moving forward.
CCNP Architecture Practice Question
This 350-401 practice question tests your understanding of architecture. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer is troubleshooting a network where OSPF neighbors are stuck in the EXSTART state. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU mismatch between the routers
The EXSTART state in OSPF indicates that routers have formed a bidirectional communication (2-Way state) and are now attempting to exchange Database Description (DBD) packets to negotiate the master/slave relationship and the initial sequence number. An MTU mismatch between the routers is the most common cause of neighbors being stuck in EXSTART because the router with the smaller MTU will drop DBD packets that exceed its interface MTU, preventing the exchange from progressing to the Loading state.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Dead timer mismatch
Why it's wrong here
Mismatched dead timers cause neighbor adjacencies to flap, not stuck in EXSTART.
✗
Authentication misconfiguration
Why it's wrong here
Authentication issues typically result in neighbors stuck in INIT state.
✗
Mismatched OSPF area IDs
Why it's wrong here
Area IDs must match for neighbors to form; mismatch would prevent reaching EXSTART.
✓
MTU mismatch between the routers
Why this is correct
A mismatch in MTU can cause OSPF to get stuck in EXSTART as DD packets are fragmented or rejected.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the EXSTART state as a symptom of MTU mismatch, but candidates frequently confuse it with authentication or area ID mismatches, which actually prevent adjacency formation at earlier stages like INIT or 2-Way.
Detailed technical explanation
How to think about this question
OSPF uses the IP MTU of the interface to determine the maximum size of DBD packets; if a router receives a DBD packet larger than its configured MTU, it silently drops it, causing the neighbor to remain in EXSTART waiting for an acknowledgment. This behavior is defined in RFC 2328, Section 10.6, and can be verified using 'show ip ospf neighbor' and 'show ip interface' commands to compare MTU values. In real-world scenarios, this often occurs when one router has an MTU of 1500 bytes and the other has 1492 bytes due to PPPoE or VPN overhead.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: MTU mismatch between the routers — The EXSTART state in OSPF indicates that routers have formed a bidirectional communication (2-Way state) and are now attempting to exchange Database Description (DBD) packets to negotiate the master/slave relationship and the initial sequence number. An MTU mismatch between the routers is the most common cause of neighbors being stuck in EXSTART because the router with the smaller MTU will drop DBD packets that exceed its interface MTU, preventing the exchange from progressing to the Loading state.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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